Chromatic aberrations of optical projection systems that utilize refractive optical elements understandably depend on the bandwidth of light used for projecting an image of a chosen object and, depending on the severity of such aberrations, may require complex designs for aberration compensation. When an optical projection system is used in cooperation with a typical lithographic scanner, a UV-light source may be utilized the spectrum of which has a bandwidth on the order of 0.1 pm (measured as full-width-half-maximum, or FWHM, value), such as an excimer laser source, for example. However, when used in cooperation with a digital scanner (such as a Spatial Light Modulator or SLM), the operation of which is modulated at rates at which the use of the excimer laser source becomes impractical, a different source of light has to be chosen. One practically appropriate choice of a laser source for use in with a maskless projection systems designed to work with a digital scanner is a laser source (such as a solid-state laser lasing at about 193 nm) the operation of which can be modulated at the rates defined by the operation of the digital scanner. The spectral bandwidth of light of laser source however, is broader, in comparison with that of the excimer laser, by about an order of magnitude (up to about 1 pm FWHM or so). While such increase in bandwidth, in turn, leads to operationally unacceptable loss of contrast of image produced by a dioptric projection system, the reduction of the working bandwidth of this laser by using, for example, appropriate optical filters has not proven effective as it leads to the substantial reduction of the irradiance at the target surface.
Attempts were made to address the loss-of-image-contrast problem by devising such projection systems that employ an SLM and that are built around the use of a projection optics containing a catadioptric sub-portion. The proposed optical projection systems can be viewed as including two main portions or sub-systems: a first portion structured as an illumination relay configured to deliver light from a light source to the SLM and to perform what in the art is referred to as “field framing”, and a second portion configured to project the light distribution from the plane of the SLM onto the image plane (the wafer) and referred to as a projection sub-system. While a projection sub-system was proposed to be structured as a catadioptric system, the illumination relay is kept conventionally dioptric (which causes, notably, an overall optical projection system to be sometimes impractically long, unless the optical path is intentionally folded with plane mirrors). It is recognized that the existing solutions still require improvements with respect to several operationally important aspects.
For example, the teachings of U.S. Pat. No. 5,523,193, while describing the basic principle of imaging of light distribution from the SLM onto the wafer, does not provide an enabling optical design for an optical projection system and, in particular, the design of an optical system that can operate at hyper-NAs that are greater than 1.0. Moreover, the described projection optics does not address the current needs in a 26-mm field size on the semiconductor wafer. Such projection optics is limited, therefore, in both the optical throughput and optical resolution that is can provide.
In U.S. Pat. No. 7,110,082, for example, an SLM-based imaging system is disclosed that utilizes a projection system with a beamsplitter in the illumination relay sub-system (to allow the on-axial illumination of the SLM without obscuration) and the projection sub-system. The described system is limited in the maximum value of the NA that can be achieved considering the practical limitations on the size of high-quality optical glass used for the fabrication of a beamsplitter of the system. In addition, the beamsplitter in this imaging system is either rather inefficient in terms of light transmission, or/and precludes the use of polarized light. The un-addressed by the related art ability of the projection system to operate in polarized light would be understandably advantageous to achieve high spatial resolution that is required to take full advantage of a hyper-NA with values exceeding 1.0.
A solution proposed in U.S. 2013/0003166 is not fully addressing an issue of compatibility of the optical projection system with a digital scanner. Indeed, while a typical digital scanner is capable of operating at high reduction ratios of 50× or, for example, even 200× (to facilitate fabrication and use of relatively geometrically large SLM pixels and reduction of the image size of such pixels to 20 nm on the semiconductor wafer), the disclosed solution enables only a relatively lower reduction ratio of 10×. In addition, U.S. 2013/0003166 is silent with respect to the design of the illumination relay portion of the overall projection system.
Accordingly, at least the greater-than-acceptable levels of chromatic aberrations in existing projection systems and insufficient reduction ratios define a need in redesign of an optical projection system for efficient use in conjunction with a digital scanner.